In a high-intensity discharge lamp, an electric arc established between two electrodes produces an intensely bright light. Such a lamp is often simply referred to as a ‘HID’ lamp. In prior art HID lamps, a discharge chamber contains a fill gas comprising mostly Xenon and a combination of halides—usually sodium iodide and scandium iodide—and one or more other metal salts that vaporize during operation of the lamp. When used in automotive headlamp applications, HID lamps have a number of advantages over other types of lamp. For instance, the light output of a metal halide xenon lamp is greater than that of a comparable tungsten-halogen lamp. Also, HID lamps have a significantly longer lifetime than filament lamps. These and other advantages make HID lamps particularly suited for automotive headlamp applications.
Along with the color temperature, other characteristics of such lamps, for example operational voltage, lamp driver characteristics, dimensions, etc., are specified in different countries by the appropriate regulations, for example by ECE-R99 in Europe, where ‘ECE’ stands for ‘Economic Commission for Europe’. Often, the lamps specified in these regulations are simply referred to by their designation, e.g. ‘D1’, ‘D4’, etc. ECE-R99 requires, for example, that the luminous flux delivered by an automotive gas-discharge headlamp be at least 2750 lm. However, continuing developments in the field of light-emitting diodes (LEDs) will eventually allow their widespread use in automotive headlamps. Since LEDs easily achieve color temperatures above 5000 K, it is to be expected that regulations governing automotive headlamps will be adjusted accordingly.
The color point, or color temperature, of an automotive lamp is crucial for safety. Firstly, the headlamps of a vehicle must sufficiently illuminate the road for the driver of that vehicle, and secondly, other drivers should not be subject to potentially dangerous glare from the headlamps of that vehicle. Furthermore, the color of the light generated by the headlamp is important since it affects the ability of the driver to distinguish objects in the path of the light beam, also referred to as color discrimination.
The color of an automotive headlight must comply with certain standards in order to ensure uniformity and therefore also to promote safety for drivers. One such standard is the SAE system, which was developed by the Society of Automotive Engineers in the USA to define the colors for automotive headlights, and which will be known to a person skilled in the art. Studies have shown that the color temperature of an automotive lamp should be considerably higher than 4000 K, and the X and Y coordinates of the corresponding color point, as graphed using the SAE system, should lie on or close to the black-body line (a locus of points corresponding to an ideal black body radiator). Such color temperature characteristics of automotive headlights improve color discrimination and also recognition of objects in the dark, therefore increasing safety in night-time driving. This is because, even at the same intensity, light with a higher color temperature—for example bluish-white light—is perceived by the human eye to be brighter than light with a lower color temperature, for example light with a yellow hue. These requirements are leading to an increased demand on the part of customers for xenon HID lamps with high efficiency mentioned, but also with a higher color temperature.
However, designing lamps to produce a bluish light is not necessarily a straightforward process, since, under equal conditions, the luminous flux output by a lamp producing blue light is lower than that of a lamp producing yellow light. For this reason, it is difficult to obtain a lamp that delivers a bright white light with a color temperature greater than 4000 K with, at the same time, an acceptable level of luminous flux. In state of the art D1 and D2 (mercury-based) lamps attempting to achieve such a high color temperature, a loss of light output up to 30% is observed, so that the efficiency of these lamps is unsatisfactory. Other D3 and D4 lamps (mercury-free) achieve a light output only marginally satisfying the regulation requirements, for example a light output of only 3200+/−450 lm.
Therefore, it is an object of the invention to provide an improved high-intensity discharge lamp with a high color temperature as well as a high luminous flux.